Evolution of Lake Uinta as Defined by Mineralogy and Geochemistry of the Green River Formation in Colorado

Abstract

Summary URTeC 1578649 Quantitative X-Ray Diffraction analysis of 117 samples, and major and trace element chemistry of 82 samples spanning the Green River Formation, representing basin margin (outcrop in Douglas Pass), and basin center (John Savage 24-1 and Shell 23X-2 cores) depositional environments reveal mineralogic and chemical relationships to the sequence stratigraphic framework of lake stages, from early fresh-brackish/mesosaline to later hypersaline conditions. Basin center samples are mudstone/marlstone composed of quartz, feldspar, and ferroan dolomite with varying clay mineral and calcite content. Basin margin samples vary from mudstone/marlstone to siltstone and fine-grained sandstone composed of quartz, feldspar, clay minerals, dolomite and calcite. Buddingtonite (NH4AlSi3O8), nahcolite (NaHCO3), dawsonite (NaAl(CO3)(OH2)), and halite (NaCl) are common in the basin center, but sparse to absent in the margin; analcime (NaAlSi2O6*H2O) is abundant in the margin, and sparse in the center. Halite and nahcolite reflect saturation in the deep lake center. Differences between basin margin and center reflect chemical stratification of the lake through its history. Ferroan dolomite, elevated V/Cr, MoEF and UEF values suggests mesosaline and dysoxic to anoxic conditions from very early in lake history. Transitional changes in mineralogy and chemistry especially in the basin center are related to important lake stage boundaries. These include: 1) sharp reduction in clay, increase in feldspar, dawsonite and Na2O content in the lower part of Stage 2, and 2) fall in quartz+dawsonite+Na2O and increase in feldspar near the top of Stage 3. Three miner-alogic units are defined in the basin center, based on the transitions, with counterparts in the basin margin. These changes are attributed to 1) shifts in diagenetic mineral stability controlled by variations in salinity and silica and CO2 activity in lake and interstitial water, and 2) changes in clastic sediment/chemical sediment ratio. The first transition reflects the change from mesosaline to hypersaline conditions. The second transition reflects increase in silica activity, leading to diagenetic feldspar formation in place of dawsonite, even as nahcolite continued to form in the lake and interstitial water. The silicate assemblage of this upper unit resembles the mineralogy of the basin margin, possibly reflecting some reduction in salinity/alkalinity before and during Stage 4.